'a' and 'w' are mutually exclusive and just 'a' is what we actually
want. Linux doesn't care (it just takes the first one) but Windows
throws a ValueError.

R=machenbach@chromium.org
BUG=None

Review-Url: https://codereview.chromium.org/2467363002
Cr-Commit-Position: refs/heads/master@{#40711}

BUG=chromium:648583
NOTRY=true

Review-Url: https://codereview.chromium.org/2365573002
Cr-Commit-Position: refs/heads/master@{#39626}

This prepares using mb as a library and not as a sub
process. It is used like that in the new list command that
shows available configurations.

Also added more robust checks of user inputs, better hints
and better docu.

BUG=chromium:625791
NOTRY=true

Review-Url: https://codereview.chromium.org/2300333002
Cr-Commit-Position: refs/heads/master@{#39136}

Helper script to generate gn arguments based on common
developer defaults or builder configurations.

BUG=chromium:625791
NOTRY=true

Review-Url: https://codereview.chromium.org/2138693002
Cr-Commit-Position: refs/heads/master@{#37947}